In many precise devices such as, for example, geodesic measuring machines, it is necessary for components or the entire machine to move. In this case, the movement must be performed precisely and, in the case of dynamic applications, also with satisfactory speed, it mostly being presumed that there are high accuracies for startup and/or repetition. In addition, the field worthiness of machines fundamentally requires robustness of the drive, low power consumption and a reliable operation within a wide temperature range.
Examples of such measuring machines are theodolites or tachymeters, which—including in conjunction with integrated automatic target acquisition and target tracking devices—are used for multifarious, measurement tasks, consideration being given both to data acquisition and a pure supervision such as, for example, in building supervision. Other measuring machines are, for example, scanner systems that use the scanning method to record topographies of surfaces as three-dimensional point clouds, or measuring machines, for example, coordinate measuring machines for high accuracy surface measurement involving contact or without contact.
In solutions known to date from the prior art, use is made for these purposes of piezo systems that provide the drive for pivoting or displacing the components. Such micromotors have spatially acting piezoelectric vibrators with contact elements as drive elements that act on an appropriately shaped running surface or friction rail and thus permit optimized movement given tuning of the piezoelectric vibrator and tuned running surface. The spatially vibrating or oscillating movement is effected by the tuned arrangement and interconnection of piezoelectric materials whose contact element, acting on the running surface, is caused, by being suitably controlled, to execute a driving, for example elliptical, movement.
Such piezoelectric systems operated in a vibrating fashion can be used in a drive for measuring machines as micromotors for various movements when running surfaces are designed in a fashion tuned to correspond to the requirements of the geodesic measuring machine. Mostly, measuring machines require both a fast movement mode with a high speed, and a mode for high precision, positioning employing the lowest speeds.
Thus, for example, WO 2006/008318 discloses a geodesic measuring machine exhibiting such a drive concept. This measuring machine has a holder for positioning the measuring machine, and a measuring component with an optical beam path for measuring a target, the measuring component being supported in an aligning component that can be moved relative to the holder and being able to be moved with respect to the aligning component. The optical beam path is modified by at least one piezo drive as a combination of the driving component with a contact element being fed and a running surface connected to the component to be adjusted.
In this case, interaction between the contact element and running surface produces a fundamentally linear movement, a feed being effected by the contact element, which makes contact with the running surface under pressure. This coupling transmits an onward driving force that permits direct positioning by the backlash-free movement. The linear feed can also be converted into rotating or rotary movements by a suitable shaping of the running surface.
Such linear or rotary drives with resonance-operated piezo ultrasound motors are currently the prior art, commercially available drives having resonance frequencies in the range of 50-200 kHz. Owing to the oscillating behavior, however, problems may arise in conjunction with a corresponding running surface geometry. Given finite running surfaces with free ends that can, in addition, be supported in a floating and damped fashion, standing waves that can influence the motor are not produced. This is different in the case of rotating drives that have annular running surfaces, or for vibrating running surface geometries with fixed ends, it being possible for the disadvantageous influences to be further increased by insufficient or nonexistent damping.
In the case of rotating drive configurations, it is mostly a ring made from hard material that is used as running surface component, the result being to reduce wear. However, at the same time it is necessary to effect a high level of friction in order to attain a good feed effect. Common materials in this case are metals with a hard layer or ceramic components. High precision movements and alignments of the components require an accurate centering of the running surface in relation to the center of rotation, that is to say typically with a deviation of between 0.01 and 0.3 mm so as to ensure a uniform running behavior. The aim in this case is for the ring and the ring suspension not to change their properties with temperature, or to do so only within certain limits. Typical environmental conditions in geodesic instrumentation that are currently valid are operating temperatures of −2.0 to +60° C., storage temperatures of −40 to 80° C., and humidities of 0-95% relative air humidity across the temperature range.
High demands result, in particular, from the fact that the ceramic component must be connected to the suspension in a fashion that is stable, accurate and unchanging with time and environmental conditions. However, problems arise from the fact that the two components—suspension and ring—have different thermal expansion behaviors. In addition, ceramic rings have only a weak and statistically strongly scattered tensile strength, although they do possess advantages in compressive strength. Moreover, power losses and noise generation and/or vibrations can result from the interaction of the piezoelectric vibrator with the running surface component, there being a dependence on the geometry of the running surface component.